Project Summary Children show remarkable limitations in their ability to suppress inappropriate actions, thoughts, and emotions. Their gradual development of inhibitory control is critical in the moment, allowing children to delay gratification, behave flexibly rather than following habits, and regulate their emotions, and in the longer term, predicting a wide range of important life outcomes, including health, academic achievement, and income. Deficits in inhibitory control are implicated in numerous clinical disorders, including ADHD, autism, OCD, and schizophrenia. Studying the development of inhibitory control thus represents an opportunity, because children's protracted limitations and successes provide a window into fundamental processes that are crucial for mental health. Studying the development of inhibitory control also represents a challenge, because targeted intervention efforts to improve children's inhibitory control have met with limited success. Some broad intervention efforts have shown promise but are multifaceted and time-intensive, making it difficult to identify which components are critical in order to inform targeted interventions and theory development. Two recent advances point to a promising way forward. The first advance is a new understanding of a core component of successful inhibitory control in adults: the ability to proactively monitor the environment for signals that indicate that an action should be stopped. This advance suggests that inhibitory limitations, developments, and interventions may depend critically upon processes for proactive monitoring. The second advance is a new understanding of developmental changes in the temporal dynamics of executive function: children gradually transition from a reactive form of executive function (engaging control processes only as needed in the moment) to a proactive form (engaging control processes and maintaining them in anticipation of needing them in the future). Together, the two advances suggest that children's struggles with inhibitory control reflect their prolonged development of proactive control (which ultimately supports the proactive monitoring that is critical for inhibitory control), and their early use of reactive control (which is less efficient for inhibitory control). Interventions must be designed accordingly. This proposal builds on our work developing the components of this framework and integrating them, to test a unified framework for understanding the development of inhibitory control and effective interventions. The studies test young children who rely heavily on reactive control, and older children who have some capacity for proactive control, and tracks children longitudinally as they progress through a key transition window. We tightly integrate these behavioral studies with computational models. Our foundational neural network simulations show how proactive monitoring can develop through learning, to support inhibitory control. Our proposed models will help to distinguish alternative theories and generate testable predictions, to advance understanding of the mechanisms supporting developments in inhibitory control and effective intervention.